1. Field of the Invention
The invention relates to a mixer, more particularly to a multiple-input mixer.
2. Description of the Related Art
Ultra wide band (UWB) communications is widely used in high data rate transmissions. Ultra wide band communications permits data to be transmitted within a wide frequency interval ranging between 3.168 GHz and 10.56 GHz. This frequency interval is sub-divided into five frequency band groups. In general, three frequency sub-bands respectively having center frequencies of 3.432 GHz, 3.960 GHz and 4.488 GHz in a first one of the frequency band groups (i.e., 3.168 GHz to 4.752 GHz) take turns in transmitting data.
FIG. 1 illustrates a conventional UWB receiver circuit 100. When an antenna 11 receives a radio frequency (RF) signal transmitted in the aforementioned manner, the RF signal is first amplified by a low-noise amplifier (LNA) 12, and then filtered by three filtering inductors 13a, 13b, 13c connected to the low-noise amplifier 12. The RF signal is subsequently outputted to one of three mixers 14a, 14b, 14c that is selected according to the frequency of the RF signal for mixing with a local oscillator (LO) signal generated by a local oscillator (LO) circuit 15 so as to generate an intermediate frequency (IF) signal provided to a rear-end demodulator circuit 16, which performs demodulation to recover data in the RF signal.
It is apparent from FIG. 1 that the conventional mixers 14a, 14b, 14c are configured to receive only one RF signal. Accordingly, when the bandwidth of the low-noise amplifier 12 is insufficient such that additional low-noise amplifiers are needed, corresponding sets of the mixers must be added, which increases overall circuit area and costs.
FIG. 2 illustrates another conventional UWB receiver circuit, which uses switches to control parallel connection of a respective capacitor 15a, 15b, 15c to a corresponding filtering inductor 13a, 13b, 13c. FIG. 3 illustrates another conventional UWB receiver circuit, which uses switches to control series connection of a respective inductor 16a, 16b, 16c to a corresponding filtering inductor 13a, 13b, 13c. Through selective connection and disconnection of a respective capacitor or a respective inductor to the filtering inductors 13a, 13b, 13c, different filtering frequency responses are generated so that two RF signals with different frequencies can be filtered in turns for subsequent output to the same mixer. However, parasitic resistances of the switches will cause signal attenuation when the switches are closed. Moreover, parasitic capacitances of the switches will result in a drift in the filtering frequency response when the switches are opened. Accordingly, signals are unable to be accurately filtered for UWB communications, thus leading to unsatisfactory signal quality.